FIG. 1 shows a configuration of a related-art passive optical network communications system defined in International Telecommunication Union-Telecommunication (ITU-T) Recommendation G.983.1. Referring to FIG. 1, the system comprises a primary station equipment unit 1, secondary station equipment units 2-1-2-n, an optical distribution network (ODN) 3 implemented by passive elements such as optical couplers and optical fibers, a primary station line termination 4 for an active system, a primary station line termination 5 for a standby system, a secondary station line termination 6 for an active system and a secondary station line termination 7 for a standby system.
In ITU-T Recommendation G.983.1, a downstream signal is broadcast from the primary station equipment unit 1 to the secondary station equipment units 2-1-2-n over the ODN 3. Upstream signals from the secondary station equipment units 2-1-2-n are multiplexed by the ODN 3 before being sent to the primary station equipment unit 1. The ODN 3 effects access control (delay control) for multiplexing the upstream signals from the secondary station equipment units 2-1-2-n. The delay control is also described in ITU-T G. 983.1.
Japanese Laid-Open Patent Application No. 11-122172, which provides a duplex configuration of an optical transmission system at a reduced cost by providing each of the primary station equipment units and secondary station equipment units with a plurality of transceivers operated at mutually different wavelengths and with a system switcher, discloses a redundant switching system for an optical subscriber system. Given in the following, however, is a description of a related-art redundant switching control in a passive optical network (PON) section that conforms to ITU-T Recommendation G.983.1.
FIG. 2 shows a detailed configuration of the related-art passive optical network communications system. Referring to FIG. 2, a secondary station equipment unit 2 is one of the secondary station equipment units 2-1-2-n. A 2-1 selector 11 selectively outputs one of a signal received by the primary station line termination 4 for the active system and the primary station line termination 5 for the standby system, under the direction of a selective control unit 13. A route setting unit 12 outputs a main signal to at least one of the primary station line termination 4 for the active system and the primary station line termination 5 for the standby system, under the direction of the selective control unit 13. The selective control unit 13 controls the 2-1 selector 11 and the route setting unit 12, based on K1/K2 bytes included in a PON section trace (PST) message, which is a type of physical layer operations administration and maintenance (PLOAM) message.
PLOAM cell composing units 14 and 15 output the PST message output from the selective control unit 13 to transceivers 18 and 19. Grant generating units 16 and 17 generate PLOAM grants (PLOAM cell transmission enable signals) so as to output the PLOAM grants thus generated to the transceivers 18 and 19. The transceivers 18 and 19 transmit the main signal, the PLOAM cell and the grant to the secondary station equipment unit 2 and also receives the main signal and the PLOAM cell from the secondary station equipment unit 2. PLOAM cell decomposing units 20 and 21 receive the PLOAM cell received by the transceivers 18 and 19 and output the PST message to the selective control unit 13.
A 2-1 selector 31 selects one of a main signal received by a secondary station line termination 6 for an active system and a main signal received by a secondary station line termination 7 for a standby system, under the direction of a selective control unit 33, so as to output the selected signal. A route setting unit 32 outputs the main signal to at least one of the secondary station line termination 6 for the active system and the secondary station line termination 7 for the standby system, under the direction of the selective control unit 33. The selective control unit 33 controls the 2-1 selector 31 and the route setting unit 32, based on the K1/K2 bytes included in the PST message.
PLOAM cell composing units 34 and 35 output the PLOAM cell including the PST message, supplied from the selective control unit 33, to the transceivers 38 and 39, the timing of output being indicated by the PLOAM grant detected by grant detecting units 36 and 37. The grant detecting units 36 and 37 detect PLOAM grants received by the transceivers 38 and 39. The transceivers 38 and 39 transmit the main signal and the PLOAM cell to the primary station equipment unit 1 and also receives the main signal, the PLOAM cell and the grant from the primary station equipment unit 1. PLOAM cell decomposing units 40 and 41 receive the main signal received by the transceivers 38 and 39 so as to output the PST message to the selective control unit 33.
FIG. 3 shows an example of control effected when a failure occurs in the secondary station equipment unit 2-1. Referring to FIG. 3, the operations of the primary station equipment unit 1 and the secondary station equipment units 2-1-2-n performed in the event of a failure are illustrated along a time line extending in a downward direction in the drawing.
A description will now be given of the operation according to the related art.
An exemplary case where a failure occurs in the secondary station equipment unit 2-1 will be described.
It is assumed that, in a state in which a failure does not occur in the secondary station equipment unit 2-1, the 2-1 selector 11 and the route setting unit 12 select the main signal from the secondary station line termination 6 for the active system and that the 2-1 selector 31 and the route setting unit 32 select the main signal from the primary station line termination 4 for the active system. Thus, the main signal is transmitted and received over the active system.
In a state in which a failure does not occur in the secondary station equipment unit 2-1, the selective control unit 33 of the secondary station equipment unit 2-1 outputs a PST message, comprising the K1 byte indicating that “switching is not requested” and the K2 byte indicating that “the active system is being selected”, to the PLOAM cell composing units 34 and 35.
The PLOAM cell including the PST message is transmitted to the primary station equipment unit 1 by the transceivers 38 and 39 of the secondary station equipment unit 2-1. The PLOAM cell thus transmitted is received by the transceivers 18 and 19 of the primary station equipment unit 1 so that the PLOAM cell decomposing units 20 and 21 output the PST message included in the PLOAM cell to the selective control unit 13.
Upon receipt of the PST message from the PLOAM cell decomposing units 20 and 21, the selective control unit 13 of the primary station equipment unit 1 controls the 2-1 selector 11 and the route setting unit 12, based on the PST message thus received.
Since the K1 byte received indicates that “switching is not requested”, the selection asserted in the 2-1 selector 11 and the route setting unit 12 is maintained (selection of the active system is maintained).
In a similar configuration as the secondary station equipment unit 2-1, the primary station equipment unit 1 also transmits the PST message, comprising the K1 byte and the K2 byte, to the secondary station equipment unit 2-1. In this example, it is assumed that the primary station equipment unit 1 does not undergo any failure so that the K1 byte indicating that “switching is not requested” continues to be transmitted to the secondary station equipment unit 2-1.
When a failure occurs in the secondary station line termination 6 for the active system in the secondary station equipment unit 2-1 (T1), the selective control unit 33 of the secondary station equipment unit 2-1 updates the PST message so as to include the K1 byte indicating that “switching is being requested due to signal fail (SF)” and the K2 byte indicating that “the active system is being selected”. The selective control unit 33 outputs the PST message thus updated to the PLOAM cell composing units 34 and 35 (T2).
The PLOAM cell including the PST message is transmitted to the primary station equipment unit 1 by the transceivers 38 and 39 of the secondary station equipment unit 2-1. The transceiver 19 of the primary station equipment unit 1 receives the PLOAM cell (the PLOAM cell does not arrive at the transceiver 18 since the failure occurred in the active system). The PLOAM cell decomposing unit 21 outputs the PST message included in the PLOAM cell to the selective control unit 13 (T3).
Upon receipt of the PST message from the PLOAM cell decomposing unit 21, the selective control unit 13 of the primary station equipment unit 1 controls the route setting unit 12, based on the PST message.
Since the K1 byte received from the secondary station equipment unit 2-1 indicates that “switching is being requested due to signal fail (SF)”, the selective control unit 13 outputs an instruction directing the route setting unit 12 to be switched to the standby system (T4).
The primary station equipment unit 1 updates the K1 byte so as to indicate “acknowledgement” (T5) since it acknowledges at this stage that a failure occurred in the secondary station equipment unit 2-1.
The selective control unit 13 of the primary station equipment unit 1 outputs the PST message, comprising the K1 byte indicating “acknowledgement” and the K2 byte indicating that “the active system is being selected”, to the PLOAM cell composing units 14 and 15.
The PLOAM cell including the PST message is transmitted by the transceivers 18 and 19 of the primary station equipment unit 1 to the secondary station equipment unit 2-1. The transceiver 39 of the secondary station equipment unit 2-1 receives the PLOAM cell. The PLOAM cell decomposing unit 41 outputs the PST message included in the PLOAM cell to the selective control unit 33 (T6).
Upon receipt of the PST message from the PLOAM cell decomposing unit 41, the selective control unit 33 of the secondary station equipment unit 2-1 controls the 2-1 selector 31 and the route setting unit 32, based on the PST message.
Since the K1 byte received indicates “acknowledgement”, the selective control unit 13 outputs an instruction directing the 2-1 selector 31 and the route setting unit 32 to be switched to the standby system (T7).
The selective control unit 33 of the secondary station equipment unit 2-1 updates the PST message so as to include the K1 byte indicating that “switching is being requested due to signal fail (SF)” and the K2 byte indicating that “the standby system is being selected” and outputs the PST message thus updated to the PLOAM cell composing units 34 and 35 (T8).
The primary station equipment unit 1 transmits PLOAM grants (PLOAM cell transmission enable signals) to the secondary station equipment units 2-1-2-n at predetermined intervals so as to receive PLOAM cells from the secondary station equipment 2-1-2-n (T0a, T0b, T0n).
ITU-T Recommendation G.983.1 recommends that the primary station equipment unit 1 and the secondary station equipment unit 2 transmit the PST message at predetermined intervals (for example, one message per second).
The selective control unit 33 of the secondary station equipment unit 2-1 outputs the PST message, comprising the K1 byte indicating that “switching is being requested due to signal fail (SF)” and the K2 byte indicating that “the standby system is being selected”, to the PLOAM cell composing units 34 and 35. Actually, the transmission of the PST message to the primary station equipment unit 1 is deferred until T71 when the PLOAM grant is received (T11).
The PLOAM cell including the above PST message is received by the transceiver 19 of the primary station equipment unit 1, where the PLOAM cell decomposing unit 21 outputs the PST message included in the PLOAM cell to the selective control unit 13.
The selective control unit 13 of the primary station equipment unit 1 receiving the PST message from the PLOAM cell decomposing unit 21 controls the 2-1 selector 11, based on the PST message.
Since the K2 byte received from the secondary station equipment unit 2-1 indicates that “the standby system is being selected”, the selective control unit 13 outputs an instruction directing the 2-1 selector 11 to be switched to the standby system (T12). Subsequently, the main signal is transmitted and received over the standby system.
Finally, the selective control unit 13 of the primary station equipment unit 1 updates the PST message so as to include the K1 byte indicating “acknowledgement” and the K2 byte indicating that “the standby system is being selected”. The selective control unit 13 outputs the PST message to the PLOAM cell composing units 14 and 15 (T13).
The PLOAM cell including the PST message is transmitted to the secondary station equipment unit 2-1 by the transceivers 18 and 19 of the primary station equipment unit (T14). The transceiver 39 of the secondary station equipment unit 2-1 receives the PLOAM cell. The PLOAM cell decomposing unit 41 outputs the PST message included in the PLOAM cell to the selective control unit 33, thus completing a redundancy switching sequence.
Referring to T71-T11, when a preceding PLOAM cell awaits transmission in the PLOAM cell inserting unit 35 of the secondary station equipment unit 2-1 before the above PST message, the PLOAM grant from the primary station equipment unit 1 is used for transmission of the PLOAM cell that is waiting. The above PST message naturally has to wait until a next PLOAM grant is received (not shown).
A description will now be given of a second example of related-art system.
FIGS. 4a and 4B show how a failure occurring in the ODN 3 coupled to the primary station equipment unit 1 is deal with. Referring to FIGS. 4A and 4B, the operations of the primary station equipment unit 1 and the secondary station equipment units 2-1-2-n occurring in the event of a failure are illustrated along a time line extending in a downward direction in the drawing.
A description will now be given of the operation according to the second example of related-art system.
An exemplary case where a failure (hereinafter, referred to as a trunk failure) occurs between an optical coupler and the primary station line termination 4 for the active system.
It is assumed that, in a state in which a trunk failure does not occur, the 2-1 selector 11 and the route setting unit 12 select the main signal from the secondary station line termination 6 for the active system and. the 2-1 selector 31 and the route setting unit 32 select the main signal from the primary station line termination 4 for the active system. Thus, the main signal is transmitted and received over the active system.
When a trunk failure occurs (T21), the primary station line termination 4 for the active system in the primary station equipment unit 1 detects a failure (for example, LOSi: loss of signal) corresponding to the secondary station equipment unit 2-1 (T22a).
The selective control unit 13 of the primary station equipment unit 1 detecting the failure in the active system outputs an instruction directing the route setting unit 12 to be switched to the standby system (T26a).
Since the failure detected corresponds to the secondary station equipment unit 2-1, the selective control unit 13 of the primary station equipment unit 1 updates the PST message so as to include the K1 byte indicating that “switching is being requested due to signal fail (SF)” and the K2 byte indicating that “the active system is being selected” (T27a).
The selective control unit 13 of the primary station equipment unit 1 outputs the PST message, comprising the K1 byte indicating that “switching is being requested due to signal fail (SF)” and the K2 byte indicating that “the active system is being selected”, to the PLOAM cell composing units 14 and 15.
The PLOAM cell including the PST message is transmitted to the secondary station equipment unit 2-1 by the transceivers 18 and 19 of the primary station equipment unit 1. The transceiver 39 of the secondary station equipment unit 2-1 receives the PLOAM cell (the PLOAM cell does not arrive at the transceiver 38 since the failure occurred in the active system). The PLOAM cell decomposing unit 41 outputs the PST message included in the PLOAM cell to the selective control unit 33 (T28a).
The selective control unit 33 of the secondary station equipment unit 2-1 receiving the PST message from the PLOAM cell decomposing unit 41 controls the 2-1 selector 31 and the route setting unit 32, based on the PST message.
Since the K1 byte received indicates that “switching is being requested due to signal fail (SF)”, the selective control unit 33 outputs an instruction directing the 2-1 selector 31 and the route setting unit 32 to be switched to the standby system (T29a-T30a).
Since the selective control unit 33 of the secondary station equipment unit 2-1 has acknowledged the detection of a failure by the primary station equipment unit 1, the selective control unit 33 updates the PST message so as to include the K1 byte indicating “acknowledgement” and the K2 byte indicating that “the standby system is being selected”. The selective control unit 33 outputs the PST thus updated to the PLOAM cell composing units 34 and 35 of the secondary station equipment unit (T31a).
The primary station equipment unit 1 transmits the PLOAM grant (PLOAM cell transmission enable signal) to the secondary station equipment units 2-1-2-n at predetermined intervals so as to receive PLOAM cells from the secondary station equipment units 2-1-2-n. 
ITU-T Recommendation G. 983.1 recommends that the primary station equipment unit 1 and the secondary station equipment units 2-1-2-n transmit the PST message at predetermined intervals (for example, one message per second).
The selective control unit 33 of the secondary station equipment unit 2-1 outputs the PST message, comprising the K1 byte indicating “acknowledgement” and the K2 byte indicating that “the standby system is being selected”, to the PLOAM cell composing units 34 and 35. Actually, the transmission of the PST message is deferred until T81a when the PLOAM grant is received (T34a).
The PLOAM cell including the above PST message is received by the transceiver 19 of the primary station equipment unit 1, where the PLOAM cell decomposing unit 21 outputs the PST message included in the PLOAM cell to the selective control unit 13 (T34a).
The selective control unit 13 of the primary station equipment unit 1 receiving the PST message from the PLOAM cell decomposing unit 21 controls the 2-1 selector 11, based on the PST message.
Since the K2 byte received indicates that “the standby system is being selected”, the selective control unit 13 outputs an instruction directing the 2-1 selector 11 to be switched to the standby system (T35a). Subsequently, the main signal is transmitted over the standby system.
Finally, the selective control unit 13 of the primary station equipment unit 1 updates the PST message so as to include the K1 byte indicating that “switching is being requested due to signal fail (SF)” and the K2 byte indicating that “the standby system is being selected”. The selective control unit 13 outputs the PST message thus updated to the PLOAM cell composing units 14 and 15 (T36a).
The PLOAM cell including the PST message is transmitted to the secondary station equipment unit 2-1 by the transceivers 18 and 19 of the primary station equipment unit (T14). The transceiver 39 of the secondary station equipment unit 2-1 receives the PLOAM cell. The PLOAM cell decomposing unit 41 outputs the PST message included in the PLOAM cell to the selective control unit 33, thus completing a redundancy switching sequence (not shown).
Since a trunk failure occurs at T21, the primary station line termination 4 for the active system in the primary station equipment unit 1 simultaneously detects failures (for example, LOSi) corresponding to the other secondary station equipment units 2-2, 2-n. The primary station equipment unit 1 performs redundancy switching corresponding to the other secondary station equipment units 2-2, 2-n (T22b-T36b, 81b, T22n-T36n, T81n) as well as that for the secondary station equipment unit 2-1 (T22a-T36a, T81a).
A description will now be given of a third example of related-art system.
FIGS. 5A and 5B show how a failure occurring in the ODN 3 coupled to the primary station equipment unit 1 is dealt with. Referring to FIGS. 5A and 5B, the operations of the primary station equipment unit 1 and the secondary station equipment units 2-1-2-n occurring in the event of a failure are illustrated along a time line extending in a downward direction in the drawing. In the third example of related-art system, the primary station line termination 4 for the active system is referred to as the primary station line termination 4 for system A and the primary station line termination 5 for the standby system is referred to as the primary system signal termination 5 for system B.
A description will now be given of the operation according to the third example of related-art system.
An exemplary case where a failure (hereinafter, referred to as a trunk failure) occurs between an optical coupler and the primary station line termination 4 for system A.
It is assumed that, in a state in which a trunk failure does not occur, the 2-1 selector 11 and the route setting unit 12 select the main signal from the secondary station line termination 6 for the active system and the 2-1 selector 31 and the route setting unit 32 select the main signal from the primary station line termination 4 for system A. Thus, the main signal is transmitted and received over system A.
When a trunk failure occurs (T21), each of the secondary station equipment units 2-1-2-n detects the failure (for example, LOS: loss of signal in the secondary station equipment unit) corresponding to the secondary station line termination 6 for the active system (T22a, T22b, T22n).
The secondary station line termination 6 of the secondary station equipment units 2-1-2-n acknowledges the failure in the active system after a predetermined guard time (TW1, TW2, TWn) so as to execute a redundant switching sequence (T23a, T23b, T23n).
A passive optical network communications system that complies with ITU-T Recommendation G. 983.1 is configured to stop the transmission to the primary station equipment unit 1 after the secondary station equipment units 2-1-2-n detect a failure in a transmission path. Therefore, the primary station equipment unit 1 also detects the transmission failure corresponding to the secondary station equipment units 2-1-2-n. At this stage, the primary station line termination 4 of system A does not detect the failure corresponding to the secondary station equipment units 2-1-2-n (T24a, T24b, T24n). Since a redundancy switching sequence is being executed in response to a request from the secondary station equipment units 2-1-2-n, the failure detected by the primary station equipment unit 1 after a predetermined period of guard time TW is neglected (T25).
Since the selective control unit 33 of the secondary station equipment unit 2-1 detected the failure in the active system, it updates the PST message so as to include the K1 byte indicating that “switching is being requested due to signal fail (SF)” and the K2 byte indicating that “the active system is being selected” (T26a).
The primary station equipment unit 1 transmits the PLOAM grant (PLOAM cell transmission enable signal) to the secondary station equipment units 2-1-2-n at predetermined intervals so as to receive the PLOAM (physical layer operation and management) cells from the secondary station equipment units 2-1-2-n. 
ITU-T Recommendation G. 983.1 recommends that the primary station equipment unit 1 and the secondary station equipment units 2-1-2-n transmit the PST message at predetermined intervals (for example, one message per second).
The selective control unit 33 of the secondary station equipment unit 2-1 outputs the PST message, comprising the K1 byte indicating that “switching is being requested due to signal fail (SF)” and the K2 byte indicating that “the active system is being selected”, to the PLOAM cell composing units 34 and 35 for the active system and the standby system, respectively. Actually, the transmission of the message by the transceivers 38 and 39 of the secondary station equipment unit 2-1 to the primary station equipment unit 1 is deferred until T27a when the PLOAM grant is received (T28a).
The transceiver 19 of the primary station equipment unit 1 receives the PLOAM cell including the PST message (since the trunk failure occurs in the active system, the PLOAM cell does not arrive at the transceiver 18). The PLOAM cell decomposing unit 21 outputs the PST message included in the PLOAM cell to the selective control unit 13.
Upon receipt of the PST message from the PLOAM cell decomposing unit 21, the selective control unit 13 of the primary station equipment unit 1 controls the route setting unit 12, based on the PST message.
Since the K1 byte received from the secondary station equipment unit 2-1 indicates that “switching is being requested due to signal fail (SF)”, the selective control unit 13 outputs an instruction directing the route setting unit 12 to be switched to system B (T29a).
Since the selective control unit 13 of the primary station equipment unit 1 has acknowledged the failure corresponding to the secondary station equipment unit 2-1, it updates the PST message so as to include the K1 byte indicating “acknowledgement” and the K2 byte indicating “system A is being selected” (T30a).
The selective control unit 13 of the primary station equipment unit 1 outputs the PST message, comprising the K1 byte indicating “acknowledgement” and the K2 byte indicating that “system A is being selected”, to the PLOAM cell composing units 14 and 15 for system A and system B, respectively.
The PLOAM cell including the PST message is transmitted by the transceivers 18 and 19 of the primary station equipment unit 1 to the secondary station equipment unit 2-1. The transceiver 39 of the secondary station equipment unit 2-1 receives the PLOAM cell. The PLOAM cell decomposing unit 41 outputs the PST message included in the PLOAM cell to the selective control unit 33 (T31a).
Upon receipt of the PST message from the PLOAM cell decomposing unit 41, the selective control unit 33 of the secondary station equipment unit 2-1 controls the 2-1 selector 31 and the route setting unit 32, based on the PST message.
Since the K1 byte received indicates “acknowledgement”, the selective control unit 33 outputs an instruction directing the 2-1 selector 31 and the route setting unit 32 to be switched to the standby system (T32a, T33a).
The selective control unit 33 of the secondary station equipment unit 2-1 updates the PST message so as to include the K1 byte indicating that “switching is being requested due to signal fail (SF)” and the K2 byte indicating that “the standby system is being selected”. The selective control unit 33 outputs the PST message thus updated to the PLOAM cell decomposing units 34 and 35 for the active system and the standby system, respectively (T34a).
The transmission of the PLOAM cell including the PST message from the transceiver 39 of the secondary station equipment 2-1 to the primary station equipment unit 1 is deferred until T35a when the PLOAM grant is received.
The PLOAM cell including the PST message is received by the transceiver 19 of the primary station equipment unit 1, where the PLOAM cell decomposing unit 21 outputs the PST message included in the PLOAM cell to the selective control unit 13 (T36a).
Upon receipt of the PST message from the PLOAM cell decomposing unit 21, the selective control unit 13 of the primary station equipment unit 1 controls the 2-1 selector 11, based on the PST message.
Since the K2 byte received indicates that “the standby system is being selected”, the selective control unit 13 outputs an instruction directing the 2-1 selector 11 to be switched to system B (T37a). Subsequently, the main signal is, transmitted and received over the standby system and system B.
Finally, the selective control unit 13 of the primary station equipment unit 1 updates the PST message so as to include the K1 byte indicating “acknowledgement” and the K2 byte indicating that “system B is being selected” to the PLOAM cell composing units 14 and 15 (T38a).
The PLOAM cell including the PST message is transmitted by the transceivers 18 and 19 of the primary station equipment to the secondary station equipment unit 2-1 (T39a). The PLOAM cell is received by the transceiver 39 of the secondary station equipment unit 2-1 where the PLOAM cell decomposing unit 41 outputs the PST message included in the PLOAM cell to the selective control unit 33, thus completing a redundancy switching sequence.
Since the trunk failure occurs at T21, the other secondary station line terminations 2-2, 2-n simultaneously detect failures (for example, LOS) corresponding to the secondary station line terminations 6-2, 6-n for the active system. In a similar configuration as the secondary station equipment unit 2-1, the secondary station equipment units 2-2, 2-n perform redundancy switching in accordance with the timing designated by the PLOAM grant from the primary station equipment unit 1 (T26b-T39b, T26n-T39n).
According to the related-art passive optical network communications systems configured as described above, the primary station equipment unit 1 transmits the PLOAM grant (PLOAM cell transmission enable signal) to the secondary station equipment units 2-1-2-n at predetermined intervals. The secondary station equipment units 2-1-2-n transmit the PST message to the primary station equipment unit 1 in accordance with the timing designated by the PLOAM grant. The configuration described above generates a queuing time in which each of the secondary station equipment units 2-1-2-n has to wait for the reception of the PLAOM grant. Thus, a relatively long period of time is required for redundancy switching.
In the redundancy switching process shown in FIGS. 4A and 4B, whenever the primary station equipment unit 1 detects a failure occurring in a trunk portion of the ODN 3 and corresponding to one of the secondary station equipment unit, redundancy, switching is executed for the secondary station equipment unit corresponding to the failure. Therefore, a relatively long period of time is required before the redundancy switching process is completed for the entirety of the secondary station equipment units 2-1-2-n. 
According to the redundancy switching process of FIGS. 5A and 5B, a failure occurring in the trunk portion of the ODN 3 is processed such that the secondary station equipment units 2-1-2-n detect the failure earlier than the primary station equipment unit 1. Therefore, the primary station equipment unit 1 responds individually to a redundancy switching request from each of the secondary station equipment units to perform individual redundancy switching processes. As a result, a relatively long period of time is required before redundancy switching is completed for the entirety of the secondary station equipment units 2-1-2-n. 
The present invention has been developed in order to resolve the aforementioned problems and has an objective of providing a passive optical network communications system in which a time required for redundancy switching for the secondary station equipment units is reduced.